1. Field of the Invention
The present invention relates to a high-frequency switch.
2. Description of Prior Art
PIN diodes are frequently employed as switches for high frequencies in the GHz range.
PIN diodes are semiconductor diodes containing a pin junction as a basic structure, which basically determines the electronical characteristics of this element.
The pin junction differs from a pn junction of a conventional semiconductor diode in particular by an intrinsic (i) layer between the p and n regions (highly doped regions p+ and n+). In the flow direction, this i layer is flooded by the two adjacent highly doped regions with holes and electrons, the result being an intense recombination in the center layer. The result is a very low forward resistance and the characteristic curve does not differ significantly from that of the pn junction. In the reverse direction, however, the entire I zone is depleted of charge carriers and the result is, compared to the pn junction, a considerably broader depletion layer and consequently a higher breakdown voltage up to several 1000 volts. A second effect of the i layer is the decrease in the depletion layer capacitance.
Due to these characteristics PIN diodes are frequently used as rectifier diodes for very high reverse voltages. Another field of application as a fast switch in the microwave range results from the low, mostly voltage-independent capacitance and the high ratio of forward resistance to reverse resistance. This is the basis for applications as microwave rectifiers, switches and as current-controllable resistors, for example for regulating HF tuners.
In particular, the attenuation in the flow operation determined by the resistance of the intrinsic zone, and the insulation depending on the depletion layer capacitance in the reverse direction are quality characteristics of the PIN diode. Intermodulation as a consequence of non-linearities is another quality characteristic.
A considerable disadvantage of the PIN diode is the considerable current flow in the forward direction. The result is that the PIN diode cannot be switched without power. The current in the forward direction is determined by injection and recombination of minority charge carriers at the highly doped p and n regions. This injection and recombination, apart from the dopant concentration of the p and n regions, particularly depends on the area thereof. This area in turn is determined by requirements as regards capacitance and series resistance.
It is another disadvantage of the PIN diode that the high-frequency (HF) path and the direct current (DC) path in the element are not separate. Further elements are consequently necessary in applications as a high-frequency switch. In particular, these elements are coupling capacitors and coils which can be realized discretely or in the form of lines.
In addition, the high-frequency signal at the PIN diode causes a small injection, following the high frequency, of charge carriers into the i zone. The resulting non-linearity determines the intermodulation performance of the diode in a negative way.